A rotary drill string used to drill wells normally experiences column loading near the drill head and tends to bend to some extent to form a curve in the axis of drill string rotation. Unless influenced by stabilizers, the curve of the drill string will allow the drill head, or bit, to produce a curve in the progressing well bore being drilled.
In many cases, those skilled in the art of well drilling will use the tendency of the drill string to curve for constructive purposes. Directional drillers place stabilizers at strategic intervals along the lower drill string assembly to urge the curve to direct the progressing drill bit along a preferred path. Such directional control has become a highly specialized field of endeavor.
Once the hole being drilled is progressing along a preferred path, whether vertical or deviated, holding the preferred path may become quite difficult. To maintain a preferred path and avoid unwanted deviation of the progressing well bore, several drill string components may be used to engage the well bore wall and use the existing well bore for guidance. Some such arrangements are called "packed hole" assemblies.
Except for the drill head, all other drill string components are obliged to be somewhat undergage to allow the drill string to move axially along the well bore to avoid jamming, or "sticking" in the hole. Downhole assemblies must allow flow space between drill string and well bore for upwardly moving drilling fluid and cutting produced by the drill head.
Even with packed hole assemblies, the necessary well bore clearance allows some curvature of the drill string. In tilted formations, the continuous drill string curvature produces continuous well bore deviations. The curved drill string tends to lie along the low side of a deviated well bore, above the drill head and may compound the problem.
Several attempts have been made to place eccentric rollers around drill string elements above the bit to engage the hole wall nearest the drill string to urge the drill string axis of rotation away from the hole near side to produce a straighter hole, when the tendency of the hole to drift from a planned path was experienced. Most notable is probably the apparatus of U.S. Pat. No. 4,220,213. Some beneficial results have been realized but the apparatus may become passive and produce no influence. The bearings in the eccentric rollers often allow the eccentric roller to become stationary at the first obstruction and fail to achieve results. Bearing resistances, or drag, used to encourage the eccentric roller to be more active do not respond to changing hole conditions and, again, the results are not predictable.
There is a need for some device to relate changing hole conditions to the tendency for the eccentric system to act a greater percentage of down hole time against the hole low side to displace the drill string centerline away from the hole low side wall.
It is therefore an object of this invention to provide apparatus to urge a rolling element mounted on a drill string interval to tend to rotate with the drill string an amount proportional to the rotating resistance encountered by the roller from the well bore wall.
It is a further object of this invention to provide a roller mounted on a drill string interval that has limited peripheral surfaces that are radially overgage to become wedged between the drill string and the well bore near side wall, yet be compelled to move some peripheral amount with each revolution of the drill string.
It is yet another object of this invention to provide a roller mounted for rotation on a drill string interval that will spend more time between the drill string and the near side of the well bore wall and turn freely with the drill string when negotiating the peripheral distance around the well bore wall farthest from the drill string.
It is still another object of this invention to provide an eccentric rolling element mounted for rotation on a drill string interval that utilizes resistance to axial motion of the roller along the well bore wall, to force the roller to move a smaller amount peripherally with each turn of the drill string, when peripheral movement of the roller is more resisted, and to move a larger amount peripherally with each turn of the drill string when peripheral movement of the roller is less resisted.
It is yet a further object of this invention to provide apparatus to space a drill string rotational centerline from a well bore wall that will reciprocate axially as the drill string rotates relatively to reduce the tendency to cause ledges on the well bore wall and to more easily negotiate existing ledges as drilling progresses.
These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached drawings and appended claims.